This invention is generally directed to telecommunication switching systems and is more specifically directed to line units associated with such systems which connect individual subscriber lines to the telecommunications system.
Telecommunications systems such as those that support the public switched telephone network employ central office systems which function as switching centers for a group of subscribers. A switching center may consist of an AT&T 5ESS.RTM. switch as shown in FIG. 1. It includes an administration module 20 which provides operation, administration and maintenance services for the switch. A communication module 22 links communication channels, i.e. different time slots which carry subscriber communications, from the switching modules. A switch module 24 switches time slots through the known time-space-time switching fabric to connect subscribers. The switching module 24 supports local line units 26 and remote line units 28 which are coupled to the switching module 24 by a communication facility 30. The function of line units 26 and 28 are to provide individual line interfaces with telephone lines coupled to customer premises equipment (CPE) 32 such as conventional telephone sets. Because of restrictions associated with the transmission of signals, line unit 26 must be located within a predetermined distance of switching module 24. The remote line unit 28 can be located farther from switching module 24 than the maximum distance permitted for a local line unit 26 by the use of a transmission facility 30 which may comprise known transmission equipment designed to transmit and receive signals over a specified transmission media. Distances between the CPE 32 and the corresponding line unit 26 or remote line unit 28 have similar maximum limits. Thus, location restrictions exist on where line units can be located relative to groups of subscribers to be served by the line unit.
A remote switching module 34 can be located beyond the maximum distance which the switching module 24 can be located relative to communications module 22 by the use of a transmission facility 36. The remote switching module 34 may support a plurality of line units, such as including line unit 38, which in turn supports CPE 32.
The location of subscribers and corresponding CPE 32 dictate the basic design requirements for a central office switch system in that either line units or remote line units must be located within the maximum distance range supported by these units. While it is possible to locate a plurality of switching modules 24 and remote switching modules 34 in a configuration relative to a centrally located communications module 22, it is desirable to minimize the number of switching modules rather than utilize "optional" switching modules in order to satisfy geographic subscriber locations in view of the costs of the switching modules. Thus, it is desirable to maximize the number of subscribers served by each switching module. This consideration requires that line units and remote line units be utilized to serve as many subscribers as possible considering the geographic and other loading considerations. A remote line unit 28 basically consists of the same components as utilized for a local line unit 26 plus additional interface capabilities to accommodate the transmission facility 30.
FIG. 2 illustrates a conventional line unit 40 which is coupled by a communication channel 42 to a switching module. The line unit includes a common shelf 44 which serves each of line group apparatus 1 through line group apparatus 16. Each of the line group apparatus is capable of connecting a plurality of subscriber lines and in the illustrative example, each line group apparatus can support up to 64 subscriber lines. Thus, each line unit 40 can support a maximum of 1028 subscriber lines. The purpose of the common shelf elements is to provide an interface to the switching module and to generate commands to control the transmission of data between a subscriber line associated with a given line group apparatus and the connected switching module.
The common shelf 44 includes a CC module 46 which includes a common control processor and a common control interface module. The CC module distributes control orders to the other elements on the common shelf and to each of the two group apparatus. A common data (CD) module 48 includes common data packs. The CD module provides the data interface path between the line group apparatus and the switching module. A high level service circuit (HLSC) 50 provides line testing, diagnostic measurements, and back-up ringing capabilities. A ring generator (RG) 52 is utilized to provide a ting signal generation capability that can be switched to any of the connected subscriber lines to thereby provide a ringing signal to the coupled CPE. A metallic access network (MAN) 54 is utilized to provide a metallic path to selected line group apparatus in order to provide paths for testing diagnostic, and ringing. Such line units and conventional line group apparatus are known in the art. It is also known that duplicate sets of the above described common shelf elements are utilized in order to provide increased reliability through redundancy so that one set can be utilized should a failure occur in the other like set of elements. It will be apparent that each of the sets of elements must have the capability of communicating with each of the line group apparatus in order to remain functional in the event of a failure to one set of elements in the common shelf. Each line group apparatus consists of a series of up to 8 line boards; each line board is capable of supporting (terminating) eight subscriber lines so that each line group apparatus can support up to 64 subscriber lines. Line boards of this type are generally known in the art.
As seen in FIG. 1 it is possible to place a remote line unit 28 further from a switching module 24 than a local line unit 26. However, each remote line unit consists of all of the elements in a local line unit plus additional interface modules to interface with the transmission facility 30. Thus, it is expensive to use a remote line unit to support a relatively small group of subscribers. Thus, there exists a need for a more flexible and cost effective means for supporting geographically dispersed subscribers.